The present invention relates to acoustically quiet gradient coil designs, particularly for use in magnetic resonance imaging (MRI).
In modern MRI scanners there is a trend for higher gradient strength and faster switching times in order to scan patients more quickly. The combination of high gradient strength and fast switching time means that the Lorentz forces which accompany the currents when switched in the presence of a high static magnetic field are very large and create correspondingly large amounts of vibration in the gradient coil structure which, in turn, generate potentially dangerous acoustic noise levels. Our first approach to ameliorate the noise problem was based on Lorentz force balancing. (Active acoustic screening: design principles for quiet gradient coils in MRI. P Mansfield, P Glover and R Bowtell, Meas Sci Technol 5, 1021-1025 (1994). Active acoustic screening: reduction of noise in gradient coils by Lorentz force balancing. P Mansfield, B L W Chapman, R Bowtell, P Glover, R Coxon and P R harvey, Mag Res Med 33, 276-281 (1995). Quiet gradient coils: active acoustically and magnetically screened distributed transverse gradient designs. B L W Chapman and P Mansfield, Meas Sci Technol 6, 349-354 (1995). Quiet transverse gradient coils: Lorentz force balanced designs using geometrical similitude. R W Bowtell and P Mansfield, Mag Res Med 34, 494-497 (1995). A quiet graident-coil set employing optimised, force-shielded, distributed coil designs. B L W Chapman and P Mansfield, J Magn Reson B107, 152-157 (1995). Analytic approach to the design of quiet transverse gradient coils. R Bowtell and P Mansfield, Proc 3rd Sci Mtg SMR, Nice, 1, 310 (1995). However, the efficacy of this method is relatively poor.
Two recent approaches which involve active acoustic cancellation of noise within the gradient structure have been proposed (Active acoustic control in quiet gradient coil design for MRI. P Mansfield, UK Patent Application 95068298.2, priority Apr. 1, 1995, PCT WO96/131785, pub. Date Oct. 10, (1996) and Active Control of Acoustic Output in Gradient Coils. P Mansfield, UK Patent Application 9620138.9, priority Sep. 27, 1996, PCT WO98/13821 Pub. Date Apr. 2, 1998) which would in principle solve the acoustic problem completely. However, the characteristics of acoustic wave propagation within the solid support structure means in practice that there are further subtleties which have to be addressed in order to approach complete noise cancellation. It is the purpose of this invention to discuss these subtleties and to propose ways forward which can be translated into practical embodiments.
U.S. Pat. No. 5,332,972 shows a bobbin structure for a gradient field magnetic field generator for MRI. The bobbin is formed such that the thickness of the axial central portion of the bobbin is thicker than that of the axial end portions for enhancing the rigidity on the axial central portion of the bobbin.
An acoustically quiet coil structure for a magnetic resonance imaging system, said coil comprising one or more electrical conductors and comprising a plate of acoustically transmitting material, said electrical conductors being either embedded directly into the plate or into a capping strip attached to the plate, said plate having a plurality of surfaces characterised in that at least one surface of said plate is provided with at least one chisel shaped,serrated or chamfered surface feature to suppress internal reflections of sound waves within the plate.
The present invention also provides an acoustically quite coil structure for a magnetic resonance imaging system, said coil comprising one or more conductors embedded in a plate of material, said plate comprising a composite cellular structure of cellular materials having positive and negative Poisson""s ratio "sgr" such that the overall "sgr" is substantially zero.